1. Field of the Invention
The present invention relates to a flexible polyurethane foam and a process for preparing the same. More particularly, the invention relates to a water blown flexible polyurethane foam which is broadly used for automotive cushion interiors, head rests and furniture cushions, and a process for preparing the flexible foam.
2. Description of the Related Art
Conventionally, flexible polyurethane foams have been prepared by the reaction of an organic polyisocyanate composition with a resin premix obtained by mixing active hydrogen containing compounds such as polyoxyalkylene polyols with blowing agents, catalysts, surfactants and other auxiliary agents.
The organic polyisocyanate composition which has been used consists of tolylene diisocyanate or a mixture of 95.about.50% by weight of tolylene diisocyanate and the remainder of polymethylenepolyphenyl isocyanate (Japanese-Laid Open Patent SHO 62-112011). Such a TDI-high resilience system (hereinafter referred to simply as TDI-HR system) which means a formulation obtained by using tolylene diisocyanate as a primary ingredient and a foam derived from the formulation, is excellent in high impact resilience of 60.about.70% and light weight. On the other hand. TDI has high vapor pressure and is liable to cause problems on working environment. TDI has also disadvantages of generating a larger amount of flash from the face of the mold in the mold filling and forming step and requiring a relatively long time for demolding.
In order to overcome these disadvantages, techniques have been proposed to use polymethylenepolyphenyl isocyanate in place of tolylene diisocyanate.
For example, Japanese Laid-Open Patent SHO 58-458 has disclosed a process for using a polyisocyanate composition consisting of 60.about.90% by weight of 4,4'-diphenylmethane diisocyanate, 3.about.30% by weight of 2,4'-diphenylmethane diisocyanate and the remainder of polymethylenepolyphenyl isocyanate having three or more benzene rings. Japanese Patent Publication SHO 63-38369 has described a process for using a urethane modified polyisocyanate composition having an NCO content of 15.about.30% which is obtained by reacting a mixture consisting of 55.about.85% by weight of diphenylmethane diisocyanate and the remainder of polymethylenepolyphenyl isocyanate having three or more benzene rings with a polyol having a functionality of 2.about.4 and a molecular weight of 60.about.1500.
Tile MDI-high resilience system (hereinafter referred to simply as MDI-HR system) obtained by using polymethylenepolyphenyl isocyanate as a primary ingredient is excellent in view of improved working environment, enhanced demolding ability and reduction of mold flash. The impact resilience is an index of riding comfort required for car cushions and it has been a serious problem that the MDI-HR system has a low impact resilience of 55.about.65% as compared with the TDI-HR system. It is also quite unfavorable that the MDI-HR system has a high vibrational characteristic value of 0.8 or more in tile transmissibility of 6 Hz vibration. Further, the above MDI-HR system used chlorofluorocarbons as auxiliary blowing agents. However, these agents cannot be used now because of the regulation in order to prevent a hostile environment due to destruction of the ozone layer. Accordingly, in the case of preparing a flexible polyurethane foam with the MDI-HR system using water as a blowing agent, the amount of water increases and a great amount of urea linkage generates.
As a result, the resulting flexible foam is inferior in physical properties such as durability (compression set) and riding comfort (impact resilience and transmissibility of 6 Hz vibration) as compared to the foam prepared by using chlorofluorocarbons as auxiliary blowing agents in combination with water. Thus, it has also been difficult to reduce tile density of the foam. Flexible polyurethane foams used for car cushions in particular are strongly required to reduce the transmissibility of the foam for 6 Hz vibration. This is because human internal organs are said to resonate to the 6 Hz vibration and thus the high transmissibility of 6 Hz vibration leads to resonance of the internal organs which provides unpleasant feeling in the course of driving the car.
In order to overcome these disadvantages, for example, Japanese Patent Publication HEI 06-021148 has disclosed a process for using a polyisocyanate composition comprising 10.about.30% by weight of compounds having three or more benzene rings in polymethylenepolyphenyl isocyanate and 1.about.10 parts by weight of tolylene diisocyanate. Japanese Laid-Open Patent HEI 04-185626 has described a process for using a polyisocyanate composition comprising 1.about.20% by weight of an isocyanate terminated prepolymer derived from tolylene diisocyanate and polyester polyol, 10.about.70% by weight of diphenylmethane diisocyanate and 89.about.10% by weight of polymethylenepolyphenyl isocyanate having three or more benzene rings.
These processes, however, have been difficult to provide a flexible polyurethane foam having an impact resilience of 60% or more, desirably 70% or more and a 6 Hz transmissibility of 0.8 or less, desirably 0.6 or less.